Rain-scatter propagation has been around for
a long time. Radar data is a way
of predicting where rain-scatter propagation can happen and/or is
happening in
real time. Here KØSM discusses how to use radar data to predict
rain-scatter
propagation. He also discussess his software program, which is designed to
be used
for making such predictions.

By Andy Flowers, KØSM

We say that an electromagnetic wave is
“scattered” when it encounters some substance in its path that deflects
some of its energy in a new direction. When one stops to think about it,
most routine propagation at VHF and higher frequencies is a result of some
sort of scattering. At VHF we often observe scattering effects from large
objects close to Earth, such as buildings and aircraft. We also know that
we can make use of small changes in air density in the lower atmosphere
that allow for routine communication of a few hundred miles with amateur
power levels. As we go higher in frequency, we find that smaller and
smaller objects have a significant effect on propagation. Raindrops become
an effective scattering medium in the microwave range. This article will
focus on the mechanics of rain-scatter propagation and how freely
available radar data can be used to predict possible propagation paths.

Scattering Principles I: Rayleigh Scattering

There are two sets of scattering equations
that are used to calculate the amount of scattering from a medium:
Rayleigh and Mie scattering. The type of scattering is a function of the
size of the scattering particle relative to the wavelength of the
radiation. Rayleigh scattering is simpler, so we will consider it first.

Rayleigh scattering applies when the diameter
of the scattering particle (d) is much smaller than the wavelength of the
radiation (l). Rayleigh scattering is the dominant scattering mode when d
< l/10. Figure 1 shows the incoming electric field from an electromagnetic
wave as it passes through a particle. When this happens, an electric
dipole (p) is induced in the particle.

The magnitude of p is given by equation 1:

K is known as Beer’s Law absorption
coefficient and is a complex number representing the scattering and
absorption properties of the dielectric. It is both wavelength and
temperature dependent. Typical values of |K|2 at 10 GHz/0°C are ~0.92 for
liquid water and ~0.19 for ice. Therefore, this confirms that ice and snow
are poorer scattering media than liquid water droplets of the same size
and shape.

Figure 1. Induction of
electric dipole in Rayleigh scattering. Arrows indicate E-field of the
incoming EM wave. The two circles show the (re-)radiation pattern
(H-plane) of the dipole.